Electrophoretic display and novel process for its manufacture

ABSTRACT

This invention relates to an electrophoretic display comprising cells which are filled with charged particles dispersed in a solvent and are individually sealed with a polymeric sealing layer which is an integral part of the display and is curved.

This application is a continuation-in-part of U.S. Ser. No. 09/518,488,filed on Mar. 3, 2000, and claims an invention directed to anelectrophoretic display comprising isolated cells filled with chargedpigment particles dispersed in a dielectric solvent. The filled cellsare individually sealed with a polymeric sealing layer.

BACKGROUND OF THE INVENTION Field of the Invention DESCRIPTION OFRELATED ART

The electrophoretic display is a non-emissive device based on theelectrophoresis phenomenon of charged pigment particles suspended in asolvent. It was first proposed in 1969. The display usually comprisestwo plates with electrodes placed opposing each other, separated byusing spacers. One of the electrodes is usually transparent. Asuspension composed of a colored solvent and charged pigment particlesis enclosed between the two plates. When a voltage difference is imposedbetween the two electrodes, the pigment particles migrate to one sideand then either the color of the pigment or the color of the solvent canbe seen according to the polarity of the voltage difference.

In order to prevent undesired movement of the particles, such assedimentation, partitions between the two electrodes were proposed fordividing the space into smaller cells. However, in the case ofpartition-type electrophoretic displays, some difficulties wereencountered in the formation of the partitions and the process ofenclosing the suspension. Furthermore, it was also difficult to keepdifferent colors of suspensions separate from each other in thepartition-type electrophoretic display.

Subsequently, attempts were made to enclose the suspension inmicrocapsules. U.S. Pat. Nos. 5,961,804 and 5,930,026 describemicroencapsulated electrophoretic displays. The reference display has asubstantially two dimensional arrangement of microcapsules each havingtherein an electrophoretic composition of a dielectric fluid and asuspension of charged pigment particles that visually contrast with thedielectric solvent. The microcapsules can be formed by interfacialpolymerization, in-situ polymerization or other known methods such asphysical processes, in-liquid curing or simple/complex coacervation. Themicrocapsules, after their formation, may be injected into a cellhousing two spaced-apart electrodes, or “printed” into or coated on atransparent conductor film. The microcapsules may also be immobilizedwithin a transparent matrix or binder that is itself sandwiched betweenthe two electrodes.

The electrophoretic displays prepared by these prior art processes, inparticular the microencapsulation process as disclosed in U.S. Pat. Nos.5,930,026, 5,961,804, and 6,017,584, have many shortcomings. Forexample, the electrophoretic display manufactured by themicroencapsulation process suffers from sensitivity to environmentalchanges (in particular sensitivity to moisture and temperature) due tothe wall chemistry of the microcapsules. Secondly the electrophoreticdisplay based on the microcapsules has poor scratch resistance due tothe thin wall and large particle size of the microcapsules. To improvethe handleability of the display, microcapsules are embedded in a largequantity of polymer matrix which results in a slow response time due tothe large distance between the two electrodes and a low contrast ratiodue to the low payload of pigment particles. It is also difficult toincrease the surface charge density on the pigment particles becausecharge-controlling agents tend to diffuse to the water/oil interfaceduring the microencapsulation process. The low charge density or zetapotential of the pigment particles in the microcapsules also results ina slow response rate. Furthermore, because of the large particle sizeand broad size distribution of the microcapsules, the prior artelectrophoretic display of this type has poor resolution andaddressability for color applications.

SUMMARY OF THE INVENTION

The first aspect of the present invention is directed to anelectrophoretic display comprising cells of well-defined shape, size andaspect ratio. The cells are filled with an electrophoretic fluidcomprising charged particles dispersed in a dielectric solvent and areindividually sealed with a polymeric sealing layer. The polymericsealing is preferably formed from a composition comprising a thermosetor thermoplastic precursor. In one embodiment of the invention, thesealing layer is an integral part of the display and is curved on theside that is in contact with the electrophoretic fluid. In anotherembodiment, both sides of the sealing layer are curved particularly ifthe electrophoretic fluid is under-filled.

Another aspect of the invention relates to a novel process for themanufacture of such an electrophoretic display.

A further aspect of the invention relates to the preparation of cells ofwell-defined shape, size and aspect ratio. The cells enclose asuspension of charged pigment particles dispersed in a dielectricsolvent and are formed from microcups prepared according to the presentinvention. Briefly, the process for the preparation of the microcupsinvolves embossing a thermoplastic or thermoset precursor layer coatedon a conductor film with a pre-patterned male mold, followed byreleasing the mold during or after the thermoplastic or thermosetprecursor layer is hardened by radiation, cooling, solvent evaporation,or other means. Alternatively, the microcups may be formed fromimagewise exposure of the conductor film coated with a radiation curablelayer followed by removing the unexposed areas after the exposed areashave become hardened.

Solvent-resistant, thermomechanically stable microcups having a widerange of size, shape, and opening ratio can be prepared by either one ofthe aforesaid methods. The microcups are then filled with a suspensionof charged pigment particles in a dielectric solvent, and sealed.

Yet another aspect of the present invention relates to the sealing ofthe microcups filled with the electrophoretic fluid containing adispersion of charged pigment particles in a dielectric fluid. Sealingcan be accomplished by a variety of ways. One of the preferredembodiments is accomplished by dispersing a sealing compositioncomprising a thermoplastics, thermoset, or their precursors in theelectrophoretic fluid before the filling step. The sealing compositionis immiscible with the dielectric solvent and has a specific gravitylower than that of the electrophoretic fluid. After filling, the sealingcomposition phase separates from the electrophoretic fluid and forms asupernatant layer at the top of the fluid. The sealing of the microcupsis then conveniently accomplished by hardening the sealing layer bysolvent evaporation, interfacial reaction, moisture, heat, or radiation.UV radiation is the preferred method to harden the sealing layer,although a combination of two or more curing mechanisms as describedabove may be used to increase the throughput of sealing.

In another preferred embodiment, the sealing can be accomplished byovercoating the electrophoretic fluid with a sealing compositioncomprising a thermoplastic, thermoset, or their precursors. The sealingis then accomplished by hardening the precursor by solvent evaporation,interfacial reaction, moisture, heat, radiation, or a combination ofcuring mechanisms. These sealing processes are especially uniquefeatures of the present invention. Additives such as surfactants,leveling agents, fillers, binders, viscosity modifiers (thinning agentsor thickeners), co-solvents, and antioxidants may be added to thesealing composition to improve the display performance. Dyes or pigmentsmay also be added in the sealing layer particularly if the display isviewed from the opposite side.

Yet another aspect of the present invention relates to a multiple stepprocess for the manufacture of a monochrome electrophoretic display. Theprocessing steps include preparation of the microcups by any one of themethods described above, sealing of the microcups, and finallylaminating the sealed array of microcups with a second conductor filmwith an adhesive. This multiple-step process can be carried out roll toroll continuously.

Yet another aspect of the present invention relates to a process for themanufacture of a full color electrophoretic display by laminating orcoating the preformed microcups with a layer of positively workingphotoresist, selectively opening a certain number of the microcups byimagewise exposing the positive photoresist, followed by developing theresist, filling the opened cups with a colored electrophoretic fluid,and sealing the filled microcups by one of the sealing processes of thisinvention. These steps may be repeated to create sealed microcups filledwith electrophoretic fluids of different colors.

These multiple-step processes as disclosed may be carried outroll-to-roll on a web continuously or semi-continuously. The microcupstructure in fact enables a format flexible and efficient roll-to-rollcontinuous manufacturing. These processes are very cost effective ascompared to typical display manufacturing processes.

One advantage of the electrophoretic display (EPD) of this invention isthat the microcup wall is in fact a built-in spacer to keep the top andbottom substrates apart at a fixed distance. The mechanical propertiesand structural integrity of this type of display is significantly betterthan any prior art displays including those manufactured by using spacerparticles. In addition, displays involving microcups have desirablemechanical properties including reliable display performance when thedisplay is bent, rolled, or under compression pressure from, forexample, a touch screen application. The use of the microcup technologyalso eliminates the need of an edge seal adhesive to predefine the sizeof the display panel and confine the display fluid inside a predefinedarea. The display fluid within a conventional display prepared by theedge sealing adhesive method will leak out completely if the display iscut in any way, or if a hole is drilled through the display. The damageddisplay will be no longer functional. In contrast, the display fluidwithin the display prepared by the microcup technology is enclosed andisolated in each cell. The microcup display may be cut into almost anydimensions without the risk of damaging the display performance due tothe loss of display fluid in the active areas. In other words, themicrocup structure enables a format flexible display manufacturingprocess, wherein the process produces a continuous output of displays ina large sheet format which can be cut into any desired sizes.

The isolated microcup or cell structure is particularly important whencells are filled with fluids of different specific properties such ascolors and switching rates. Without the microcup structure, it will bevery difficult to prevent the fluids in adjacent areas from intermixingor being subject to cross-talk during operation. As a result, thebistable reflective display of this invention also has excellent coloraddressability and switching performances.

The electrophoretic display prepared according to the present inventionis not sensitive to environment, particularly humidity and temperature.The display is thin, flexible, durable, easy-to-handle, andformat-flexible. The drawbacks of electrophoretic displays prepared bythe prior art processes are therefore all eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of the electrophoretic display of thepresent invention.

FIGS. 2a and 2 b show the roll-to-roll process for the manufacture of anelectrophoretic display, in particular, the creation of microcups byembossing a conductor film coated with a UV curable composition.

FIGS. 3a-3 d illustrate a typical method of preparing the male mold formicroembossing.

FIGS. 4a-4 c show a typical microcup array prepared by microembossing.

FIGS. 5a-5 c show alternative processing steps for preparing themicrocups involving imagewise exposure of the conductor film coated witha thermoset precursor to UV radiation.

FIG. 6 is a flow chart for manufacturing a black/white electrophoreticdisplay or other single color electrophoretic displays.

FIGS. 7a-7 h are a flow chart for manufacturing a multi-colorelectrophoretic display.

FIGS. 8a-8 c depict an electrophoretic display cell sealed with asealing layer of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Definitions

Unless defined otherwise in this specification, all technical terms areused herein according to their conventional definitions as they arecommonly used and understood by those of ordinary skill in the art.

The term “microcup” refers to the cup-like indentations created bymicroembossing or imagewise exposure.

The term “cell”, in the context of the present invention, is intended tomean the single unit formed from a sealed microcup. The cells are filledwith charged pigment particles dispersed in a solvent or solventmixture.

The term “well-defined”, when describing the microcups or cells, isintended to indicate that the microcup or cell has a definite shape,size and aspect ratio which are pre-determined according to the specificparameters of the manufacturing process.

The term “aspect ratio” is a commonly known term in the art ofelectrophoretic displays. In this application, it refers to the depth towidth or depth to length ratio of the microcups.

The term “integral part” is intended to mean that a particular elementof the display is physically or chemically bonded to another element orother elements of the display.

Preferred Embodiments

An electrophoretic display of the present invention, as shown in FIG. 1,comprises two electrode plates (10, 11), at least one of which istransparent (10), and a layer of well-defined cells (12) enclosedbetween the two electrodes. The cells are filled with charged pigmentparticles dispersed in a colored dielectric solvent, and individuallysealed with a polymeric sealing layer (not shown). When a voltagedifference is imposed between the two electrodes, the charged particlesmigrate to one side, such that either the color of the pigment or thecolor of the solvent is seen through the transparent conductor film. Atleast one of the two conductors is patterned. The process for thepreparation of such an electrophoretic display involves several aspects.

I. Preparation of the Microcups

I(a) Preparation of the Microcups by Embossing

This processing step is shown in FIGS. 2a and 2 b. The male mold (20)may be placed either above (FIG. 2a) or below (FIG. 2b) the web (24).The transparent conductive substrate is constructed by forming atransparent conductor film (21) on a glass plate or a plastic substrate.A composition comprising a thermoplastic, thermoset, or their precursors(22) is then coated on the conductor film. The thermoplastic orthermoset precursor layer is embossed at a temperature higher than theglass transition temperature of the thermoplastics or thermosetprecursor layer by the male mold in the form of a roller, plate or belt.

The thermoplastics or thermoset precursor for the preparation of themicrocups may be multifunctional acrylate or methacrylate, vinylether,epoxide and their oligomers, polymers and the like. Multifunctionalacrylate and their oligomers are the most preferred. A combination ofmultifunctional epoxide and multifunctional acrylate is also very usefulto achieve desirable physico-mechanical properties. A crosslinkableoligomer imparting flexibility, such as urethane acrylate or polyesteracrylate, is usually also added to improve the flexure resistance of theembossed micro-cups. The composition may contain polymer, oligomer,monomer and additives or only oligomer, monomer and additives. The glasstransition temperatures (or Tg) for this class of materials usuallyrange from about −70° C. to about 150° C., preferably from about −20° C.to about 50° C. The microembossing process is typically carried out at atemperature higher than the Tg. A heated male mold or a heated housingsubstrate against which the mold presses may be used to control themicroembossing temperature and pressure.

As shown in FIGS. 2a and 2 b, the mold is released during or after theprecursor layer is hardened to reveal an array of microcups (23). Thehardening of the precursor layer may be accomplished by cooling, solventevaporation, cross-linking by radiation, heat or moisture. If the curingof the thermoset precursor is accomplished by UV radiation, UV mayradiate onto the transparent conductor film from the bottom or the topof the web as shown in the two figures. Alternatively, UV lamps may beplaced inside the mold. In this case, the mold must be transparent toallow the UV light to radiate through the pre-patterned male mold on tothe thermoset precursor layer.

Preparation of the Male Mold

The male mold may be prepared by a photoresist process followed byeither etching or electroplating. A representative example for thepreparation of the male mold is given in FIG. 3. With electroplating(FIG. 3a), a glass base (30) is sputtered with a thin layer (typically3000 Å) of a seed metal (31) such as chrome inconel. It is then coatedwith a layer of photoresist (32) and exposed to UV. A mask (34) isplaced between the UV and the layer of photoresist (32). The exposedareas of the photoresist become hardened. The unexposed areas are thenremoved by washing them with an appropriate solvent. The remaininghardened photoresist is dried and sputtered again with a thin layer ofseed metal. The master (FIG. 3b) is then ready for electroforming. Atypical material used for electroforming is nickel cobalt (33).Alternatively, the master can be made of nickel by nickel sulfamateelectroforming or electroless nickel deposition as described in“Continuous manufacturing of thin cover sheet optical media”, SPIE Proc.Vol. 1663, pp.324 (1992). The floor of the mold (FIG. 3d) is typicallybetween 50 to 400 microns thick. The master can also be made using othermicroengineering techniques including e-beam writing, dry etching,chemical etching, laser writing or laser interference as described in“Replication techniques for micro-optics”, SPIE Proc. Vol. 3099, pp76-82(1997). Alternatively, the mold can be made by photomachining usingplastics, ceramics or metals.

FIG. 4a is an optical profilometry three-dimensional profile of thetypical microcups prepared by microembossing. FIG. 4b is an opticalmicroscopic picture showing the openings of the microcups from the topview. FIG. 4c is the optical profilometry vertical cross-section view ofa row of microcups showing their depth.

I(b) Preparation of the Microcups by Imagewise Exposure

Alternatively, the microcups may be prepared by imagewise exposure (FIG.5a) of a radiation curable material (51) coated on the conductor film(52) to UV or other forms of radiation through a mask (50). Theconductor film (52) is on a plastic substrate (53).

For a roll-to-roll process, the photomask may be synchronized with theweb and move at the same speed as the latter. In the photomask (50) inFIG. 5a, the dark squares (54) represent the opaque area and the space(55) between the dark squares represents the opening area. The UVradiates through the opening area (55) onto the radiation curablematerial. The exposed areas become hardened and the unexposed areas(protected by the opaque area in the mask) are then removed by anappropriate solvent or developer to form the microcups (56). The solventor developer is selected from those commonly used for dissolving orreducing the viscosity of radiation curable materials such asmethylethylketone, toluene, acetone, isopropanol or the like.

FIGS. 5b and 5 c illustrate two other options for the preparation ofmicrocups by imagewise exposure. The features in these two figures areessentially the same as shown in FIG. 5a and the corresponding parts arealso numbered the same. In FIG. 5b, the conductor film (52) used isopaque and pre-patterned. In this case, it may be advantageous toimagewise expose the radiation sensitive material through the conductorpattern which serves as the photomask. The microcups (56) can then beformed by removing the unexposed areas after UV radiation. In FIG. 5c,the conductor film (52) is also opaque and line-patterned. The radiationcurable material is exposed from the bottom through the conductor linepattern (52) which serves as the first photomask. A second exposure isperformed from the other side through the second photomask (50) having aline pattern perpendicular to the conductor lines. The unexposed area isthen removed by a solvent or developer to reveal the microcups (56).

In general, the microcups can be of any shape, and their sizes andshapes may vary. The microcups may be of substantially uniform size andshape in one system. However, in order to maximize the optical effect,microcups having a mixture of different shapes and sizes may beproduced. For example, microcups filled with a dispersion of the redcolor may have a different shape or size from the green microcups or theblue microcups. Furthermore, a pixel may consist of different numbers ofmicrocups of different colors. For example, a pixel may consist of anumber of small green microcups, a number of large red microcups, and anumber of small blue microcups. It is not necessary to have the sameshape and number for the three colors.

The openings of the microcups may be round, square, rectangular,hexagonal, or any other shape. The partition area between the openingsis preferably kept small in order to achieve a high color saturation andcontrast while maintaining desirable mechanical properties. Consequentlythe honeycomb-shaped opening is preferred over, for example, thecircular opening.

For reflective electrophoretic displays, the dimension of eachindividual microcup may be in the range of about 10² to about 5×10⁵ μm²,preferably from about 10³ to about 5×10⁴ μm². The depth of the microcupsis in the range of about 3 to about 100 microns, preferably from about10 to about 50 microns. The ratio between the area of opening to thearea of cell walls is in the range of from about 0.05 to about 100,preferably from about 0.4 to about 20. The width of the openings usuallyare in the range of from about 15 to about 450 microns, preferably fromabout 25 to about 300 microns from edge to edge of the openings.

II. Preparation of the Suspension/Dispersion

The microcups are filled with charged pigment particles dispersed in adielectric solvent. The dispersion may be prepared according to methodswell known in the art such as U.S. Pat. Nos. 6,017,584, 5,914,806,5,573,711, 5,403,518, 5,380,362, 4,680,103, 4,285,801, 4,093,534,4,071,430, 3,668,106 and IEEE Trans. Electron Devices, ED-24, 827(1977), and J. Appl. Phys. 49(9), 4820 (1978). The charged pigmentparticles visually contrast with the medium in which the particles aresuspended. The medium is a dielectric solvent which preferably has a lowviscosity and a dielectric constant in the range of about 1 to about 30,preferably about 1.5 to about 15 for high particle mobility. Examples ofsuitable dielectric solvents include hydrocarbons such asdecahydronaphthalene (DECALIN), 5-ethylidene-2-norbornene, fatty oils,paraffin oil, aromatic hydrocarbons such as toluene, xylene,phenylxylylethane, dodecylbenzene and alkylnaphthalene, halogenatedsolvents such as perfluorodecalin, perfluorotoluene, perfluoroxylene,dichlorobenzotrifluoride, 3,4,5-trichlorobenzotrifluoride,chloropentafluoro-benzene, dichlorononane, pentachlorobenzene, andperfluoro solvents such as FC-43™, FC-70™ and FC-5060™ from 3M Company,St. Paul Minn., low molecular weight halogen containing polymers such aspoly(perfluoropropylene oxide) from TCI America, Portland, Oreg.,poly(chlorotrifluoroethylene) such as Halocarbon Oils from HalocarbonProduct Corp., River Edge, N.J., perfluoropolyalkylether such as Galden™from Ausimont or Krytox™ Oils and Greases K-Fluid Series from DuPont,Del. In one preferred embodiment, poly(chlorotrifluoroethylene) is usedas the dielectric solvent. In another preferred embodiment,poly(perfluoropropylene oxide) is used as the dielectric solvent.

In addition to the charged primary pigment particles such as TiO₂particles, the electrophoretic fluid may be colored by a contrastingcolorant. The contrast colorant may be formed from dyes or pigments.Nonionic azo, anthraquinone and phthalocyanine dyes or pigments areparticularly useful. Other examples of useful dyes include, but are notlimited to: Oil Red EGN, Sudan Red, Sudan Blue, Oil Blue, Macrolex Blue,Solvent Blue 35, Pylam Spirit Black and Fast Spirit Black from PylamProducts Co., Arizona, Sudan Black B from Aldrich, Thermoplastic BlackX-70 from BASF, anthraquinone blue, anthraquinone yellow 114,anthraquinone reds 111 and 135 and anthraquinone green 28 from Aldrich.In case of an insoluble pigment, the pigment particles for generatingthe color of the medium may also be dispersed in the dielectric medium.These color particles are preferably uncharged. If the pigment particlesfor generating color in the medium are charged, they preferably carry acharge which is opposite from that of the charged pigment particles. Ifboth types of pigment particles carry the same charge, then they shouldhave different charge density or different electrophoretic mobility. Inany case, the dye or pigment for generating color of the medium must bechemically stable and compatible with other components in thesuspension.

The charged pigment particles may be organic or inorganic pigments, suchas TiO₂, phthalocyanine blue, phthalocyanine green, diarylide yellow,diarylide AAOT Yellow, and quinacridone, azo, rhodamine, perylenepigment series from Sun Chemical, Hansa yellow G particles from KantoChemical, and Carbon Lampblack from Fisher. Submicron particle size ispreferred. The particles should have acceptable optical characteristics,should not be swollen or softened by the dielectric solvent, and shouldbe chemically stable. The resulting suspension must also be stableagainst sedimentation, creaming or flocculation under normal operatingconditions.

The pigment particles may exhibit a native charge, or may be chargedexplicitly using a charge control agent, or may acquire a charge whensuspended in the dielectric solvent. Suitable charge control agents arewell known in the art; they may be polymeric or non-polymeric in nature,and may also be ionic or non-ionic, including ionic surfactants such asAerosol OT, sodium dodecylbenzenesulfonate, metal soap, polybutenesuccinimide, maleic anhydride copolymers, vinylpyridine copolymers,vinylpyrrolidone copolymer (such as Ganex™ from International SpecialtyProducts), (meth)acrylic acid copolymers, and N,N-dimethylaminoethyl(meth)acrylate copolymers. Fluorosurfactants are particularly useful ascharge controlling agents in fluorocarbon solvents. These include FCfluorosurfactants such as FC-170C™, FC-171™, FC-176™, FC430™, FC431™ andFC-740™ from 3M Company and Zonyl™ fluorosurfactants such as Zonyl™ FSA,FSE, FSN, FSN-100, FSO, FSO-100, FSD and UR from Dupont.

Suitable charged pigment dispersions may be manufactured by any of thewell-known methods including grinding, milling, attriting,microfluidizing, and ultrasonic techniques. For example, pigmentparticles in the form of a fine powder are added to the suspendingsolvent and the resulting mixture is ball milled or attrited for severalhours to break up the highly agglomerated dry pigment powder intoprimary particles. Although less preferred, a dye or pigment forgenerating color of the suspending medium may be added to the suspensionduring the ball milling process.

Sedimentation or creaming of the pigment particles may be eliminated bymicroencapsulating the particles with suitable polymers to match thespecific gravity to that of the dielectric solvent. Microencapsulationof the pigment particles may be accomplished chemically or physically.Typical microencapsulation processes include interfacial polymerization,in-situ polymerization, phase separation, coacervation, electrostaticcoating, spray drying, fluidized bed coating and solvent evaporation.

For a black/white electrophoretic display, the suspension comprisescharged white particles of titanium oxide (TiO₂) dispersed in a blacksolvent or charged black particles dispersed in a dielectric solvent. Ablack dye or dye mixture such as Pylam™ Spirit Black and Fast SpiritBlack from Pylam Products Co. Arizona, Sudan Black B from Aldrich,Thermoplastic Black X-70™ from BASF, or an insoluble black pigment suchas carbon black may be used to generate the black color of the solvent.Carbonaceous particles, particularly submicron carbonaceous particles,prepared from organic compounds such as coal tar, petroleum pitch orresins by a high temperature carbonizing process as taught in U.S. Pat.Nos. 5,332,517 and 5,693,367 may also be used as the black colorant. Forother colored suspensions, there are many possibilities. For asubtractive color system, the charged TiO₂ particles may be suspended ina dielectric solvent of cyan, yellow or magenta color. The cyan, yellowor magenta color may be generated via the use of a dye or a pigment. Foran additive color system, the charged TiO₂ particles may be suspended ina dielectric solvent of red, green or blue color generated also via theuse of a dye or a pigment. The red, green, blue color system ispreferred for most applications.

III. Sealing of the Microcups

The sealing of the microcups may be accomplished in a number of ways. Apreferred approach is to disperse into an electrophoretic fluid asealing composition comprising a material selected from the groupconsisting of polyvalent acrylate or methacrylate, cyanoacrylates,polyvalent vinyl including vinylbenzene, vinylsilane, vinylether,polyvalent epoxide, polyvalent isocyanate, polyvalent allyl, oligomersor polymers containing crosslinkable functional groups and the like, andoptionally additives such as a polymeric binder or thickener,photoinitiator, catalyst, filler, colorant, and surfactant. The sealingcomposition and the electrophoretic fluid containing charged pigmentparticles dispersed in a colored dielectric solvent are thoroughlyblended by, for example, an in-line mixer and immediately coated ontothe microcups with a precision coating mechanism such as Myrad bar,gravure, doctor blade, slot coating or slit coating. If desired, excessfluid is scraped away by a wiper blade or a similar device. A smallamount of a weak solvent or solvent mixture such as isopropanol,methanol or an aqueous solution thereof may be used to clean theresidual electrophoretic fluid on the top surface of the partition wallsof the microcups. The sealing composition is immiscible with thedielectric solvent and has a specific gravity lower than that of thedielectric solvent and the pigment particles. Volatile organic solventsmay be used to control the viscosity and coverage of the electrophoreticfluid. The thus-filled microcups are then dried and the sealingcomposition floats to the top of the electrophoretic fluid. Themicrocups may be sealed by hardening the supernatant sealing layer byfor example, UV, during its separation or after it floats to the top.Other forms of radiation such as visible light, IR and electron beam maybe used to cure and seal the microcups. Alternatively, heat or moisturemay also be employed to dry, harden and seal the microcups, if heat ormoisture curable compositions were used.

A preferred group of dielectric solvents exhibiting desirable densityand solubility discrimination against acrylate monomers and oligomersare halogenated hydrocarbons particularly fluorinated and perfluorinatedsolvents and their derivatives. Surfactants may be used to improve theadhesion and wetting at the interface between the electrophoretic fluidand the sealing materials. Useful surfactants include the FC™surfactants from 3M Company, Zonyl™ fluorosurfactants from DuPont,fluoroacrylates, fluoromethacrylates, fluoro-substituted long chainalcohols, perfluoro-substituted long chain carboxylic acids and theirderivatives.

Alternatively, the electrophoretic fluid and the sealing composition maybe coated sequentially into the microcups, if the sealing precursor isat least partially compatible with the dielectric solvent. Thus, thesealing of the microcups may be accomplished by overcoating a thin layerof a sealing composition comprising a material selected from the groupconsisting of polyvalent acrylate or methacrylate, cyanoacrylates,polyvalent vinyl including vinylbenzene, vinylsilane, vinylether,polyvalent epoxide, polyvalent isocyanate, polyvalent allyl, oligomersor polymers containing crosslinkable functional groups and the like. Thematerial may be curable by radiation, heat, moisture or interfacialreactions and curing on the surface of the filled microcups. Interfacialpolymerization followed by UV curing is very beneficial to the sealingprocess. Intermixing between the electrophoretic layer and the overcoatis significantly suppressed by the formation of a thin barrier layer atthe interface by interfacial polymerization. The sealing is thencompleted by a post curing step, preferably by UV radiation. To furtherreduce the degree of intermixing, it is highly desirable that thespecific gravity of the overcoating is significantly lower than that ofthe electrophoretic fluid. Volatile organic solvents may be used toadjust the viscosity and the thickness of the coatings. When a volatilesolvent is used in the overcoat, it is preferred that it is immisciblewith the dielectric solvent. The two-step overcoating process isparticularly useful when the colorant used in the electrophoretic fluidis at least partially compatible with the sealing composition. Additivesor fillers such as surfactants, antioxidants, crosslinkers, thickeners,and polymer binders may also be used to improve the performance orprocessability. Pigments, dyes, or fillers such as silica, CaCO₃, BaSO₄,TiO₂, metal particles and their oxides, carbon black, may also be usedparticularly when the display is viewed from the opposite side.

The sealing layer forms a contiguous film on the electrophoreticdispersion and is an integral part of the electrophoretic display. Inone embodiment of the invention, the sealing layer is curved on the sidethat is in contact with the dispersion. In another embodiment, bothsides of the sealing layer may be curved. The sealing layer curvestowards the dispersion and it may be of the U shape or the W-shape withflattened and elongated center portion.

FIG. 8a shows a display cell (80) wherein the sealing layer (81) isphysically or chemically bonded to the top of the side walls (82) of thecell and the layer is curved on the side (84) which is in contact withthe dispersion fluid (83). In FIG. 8b, the curved sealing layer (81) isin intimate contact with both the fluid and the inner surface (85) ofthe walls (82) of the cell that are not covered by the fluid. Inaddition, the sealing layer may extend over the top of the side walls ofthe cells. FIG. 8c shows a sealing layer (81) having both sides curved.

The cell is sandwiched between two conductive layers (86 and 87). Theremay be an additional adhesive layer (88) between the top of the sealinglayer (81) and the top conductive layer (86). The application of the topconductive layer and the adhesive layer to the cell is illustrated inthe following sections.

IV. Preparation of Monochrome Electrophoretic Displays

The process is illustrated by the flow diagram as shown in FIG. 6. Allmicrocups are filled with a suspension of the same color composition.The process can be a continuous roll-to-roll process comprising thefollowing steps:

1. Coat a layer of thermoplastic, thermoset, or their precursors (60)optionally with a solvent on a conductor film (61). The solvent, ifpresent, readily evaporates.

2. Emboss the layer (60) at a temperature higher than the glasstransition temperature of the layer by a pre-patterned male mold (62).

3. Release the mold from the layer (60) preferably during or after it ishardened by proper means.

4. Fill in the thus-formed array of microcups (63) with a chargedpigment dispersion (64) in a colored dielectric solvent containing asealing composition which is incompatible with the solvent and has alower specific gravity than the electrophoretic fluid.

5. Seal the microcups by hardening the sealing composition preferably byradiation such as UV (65), or by heat or moisture during or after thesealing composition separates and forms a supernatant layer on top ofthe liquid phase, thus forming closed electrophoretic cells containingelectrophoretic fluid.

6. Laminate the sealed array of electrophoretic cells with a secondconductor film (66) pre-coated with an adhesive layer (67) which may bea pressure sensitive adhesive, a hot melt adhesive, a heat, moisture, orradiation curable adhesive. Preferred materials for the adhesive includeacrylics, styrene-butadiene copolymers, styrene-butadiene-styrene blockcoplymers, styrene-isoprene-styrene block copolymers, polyvinylbutyal,cellulose acetate butyrate, polyvinylpyrrolidone, polyurethanes,polyamides, ethylene-vinylacetate copolymers, epoxides, multifunctionalacrylates, vinyls, vinylethers, and their oligomers, polymers, andcopolymers.

The laminated adhesive may be post cured by radiation such as UV (68)through the top conductor film if the latter is transparent to theradiation. The finished product may be cut (69) after the laminationstep.

The preparation of the microcups described above can be convenientlyreplaced by the alternative procedure of imagewise exposing theconductor film coated with a radiation curable composition followed byremoving the unexposed areas by an appropriate solvent.

In one of the preferred embodiment of the invention, the sealing of themicrocups may alternatively be accomplished by first partially fillingthe microcup array with the electrophoretic fluid and then directlyovercoating and hardening the sealing composition over the surface ofthe fluid. This two-step overcoating sealing process is particularlyuseful when the colorant of the electrophoretic fluid is at leastpartially compatible with the sealing composition.

V. Preparation of Multi-Color Electrophoretic Displays

For the manufacture of a multi-color electrophoretic display, additionalsteps are needed to generate microcups containing suspensions ofdifferent colors. These additional steps include (1) laminating thealready formed microcups with a positively working dry-film photoresistconsisting of at least a removable support such as PET-4851™ fromSaint-Gobain, Worcester, Mass., a novolac positive photoresist such asMicroposit S1818™ from Shipley, and an alkali-developable adhesive layersuch as a mixture of Nacor 72-8685™ from National Starch and Carboset515™ from BF Goodrich; (2) selectively opening a certain amount of themicrocups by imagewise exposing the photoresist, removing the removablesupport film, and developing the positive photoresist with a developersuch as diluted Microposit 351™ developer from Shipley; (3) filling theopened cups with the electrophoretic fluid containing charged whitepigment (TiO₂) particles and dye or pigment of the first primary color;and (4) sealing the filled microcups as described in the preparation ofmonochrome displays. These additional steps may be repeated to createmicrocups filled with electrophoretic fluid of the second and the thirdprimary colors. Alternatively, the positively working resist may beapplied to the unfilled microcup array by a conventional wet coatingprocess.

More specifically, a multi-color electrophoretic displays may beprepared according to the steps as shown in FIG. 7:

1. Coat a layer of thermoplastic, thermoset, or their precursors (70) ona conductor film (71).

2. Emboss the layer (70) at a temperature higher than its glasstransition temperature by a pre-patterned male mold (not shown).

3. Release the mold from the layer (70) preferably during or after it ishardened by solvent evaporation, cooling or crosslinking by radiation,heat or moisture.

4. Laminate the thus formed array of microcups (72) with a dry filmpositive photoresist which comprises at least an adhesive layer (73), apositive photoresist (74) and a removable plastic cover sheet (notshown).

5. Imagewise expose (FIG. 7c) the positive photoresist by UV, visiblelight, or other radiation, remove the cover sheet, develop and open cupsin the exposed area. The purpose of Steps 4 and 5 is to selectively openthe microcups in a predetermined area (FIG. 7d).

6. Fill in the opened microcups with a charged white pigment dispersion(75) in a dielectric solvent containing at least a dye or pigment of thefirst primary color and a sealing composition (76) which is incompatiblewith the electrophoretic fluid and has a lower specific gravity than thesolvent or the pigment particles.

7. Seal the microcups to form closed electrophoretic cells containingelectrophoretic fluid of the first primary color by hardening thesealing composition (preferably by radiation such as UV, less preferablyby heat or moisture) during or after the sealing composition separatesand forms a supernatant layer on top of the electrophoretic fluid (FIG.7e).

8. Steps 5-7 described above may be repeated to generate well-definedcells containing electrophoretic fluids of different colors in differentareas (FIGS. 7e, 7 f and 7 g).

9. Laminate the sealed array of electrophoretic cells in registration toa second, pre-patterned transparent conductor film (77) pre-coated withan adhesive layer (78) which may be a pressure sensitive adhesive, a hotmelt adhesive, a heat, moisture, or radiation curable adhesive.Preferred materials for the adhesive include acrylics, styrene-butadienecopolymers, styrene-butadiene-styrene block copolymers,styrene-isoprene-styrene block copolymers, polyvinylbutyral, celluloseacetate butyrate, polyvinylpyrrolidone, polyurethanes, polyamides,ethylene-vinylacetate copolymers, epoxides, multifunctional acrylates,vinyls, vinylethers, and their oligomers, polymers, and copolymers.

10. Harden the adhesive.

The preparation of the microcups described in the process above canconveniently be replaced by the alternative procedure of imagewiseexposing the conductor film coated with a radiation curable compositionfollowed by removing the unexposed areas by an appropriate solvent. Thesealing of the microcups may be alternatively accomplished by directlycoating a layer of the sealing composition over the surface of theliquid phase. The positively working photoresist in Step 4 mayalternatively be applied onto the unfilled microcup array by aconventional wet coating process.

The thickness of the display produced by the present processes asdescribed can be as thin as a piece of paper. The width of the displayis the width of the coating web (typically 3-90 inches). The length ofthe display can be anywhere from inches to thousands of feet dependingon the size of the roll.

EXAMPLES

The following examples are given to enable those skilled in the art tomore clearly understand and to practice the present invention. Theyshould not be considered as limiting the scope of the invention, butmerely as being illustrative and representative thereof.

Example 1

Preparation of Microcups by Microembossing

The composition shown in Table 1 was coated onto Mylar™ J101/200 gaugeusing a Nickel Chrome bird type film applicator with an opening of 3mil. The solvent was allowed to evaporate leaving behind a tacky filmwith a Tg below room temperature.

TABLE 1 PMMA-containing composition for microembossing No. DescriptionIngredient Supplier Wt % 1 Epoxy acrylate Ebecryl ™ 3605 UCB 7.35Chemicals 2 Monomer Sartomer ™ Sartomer 9.59 SR205 3 Urethane acrylateEbecryl ™ 6700 UCB 4.87 Chemicals 4 Polymethylmethacrylate Elvacite ™2051 ICI 9.11 5 Photoinitiator Darocur ™ 1173 Ciba 1.45 6 Cationicphotoinitiator Cyracure ™ UVI Union 0.60 6976 Carbide 7 Solvent AcetoneAldrich 67.03 Total 100.00

A pre-patterned stencil from Photo Stencil, Colorado Springs, Colo., wasused as the male mold for microembossing and Frekote™ 700-NC from Henkelwas used as the mold release. The coated film was then embossed by thestencil using a pressure roller at room temperature. The coating wasthen UV cured for about 20 minutes through the Mylar™ film using aLoctite Zeta 7410™ exposure unit equipped with a metal fluoride lampwith an intensity of 80 mW/cm² at 365 nm. The embossed film was thenreleased from the mold to reveal well-defined microcups having lateraldimensions ranging from 60 μm to 120 μm (200-400 dpi) and a depthranging from 5 μm to 30 μm as measured by optical profilometry andmicroscope (FIGS. 4a-4 c).

Example 2

Preparation of Microcups

A composition containing solid oligomer, monomer and additive is shownin Table 2. The glass transition temperature of the mixture is againbelow room temperature. The tacky coating was deposited on top of Mylar™J101/200 gauge as before. Embossing was conducted at 60° C. using aheated pressure roller or laminator. Well-defined high resolutionmicrocups (100-400 dpi) with depth ranging from 5-30 microns wereproduced.

TABLE 2 Embossing composition containing oligomer, monomer, additive andsolvent No. Description Ingredient Supplier Wt % 1 Epoxy acrylateEbecry ™ I 3903 UCB Chemicals 17.21 2 Monomer HDODA UCB Chemicals 8.61 3Urethane acrylate Ebecryl ™ 4827 UCB Chemicals 2.87 4 PhotoinitiatorIrgacure ™ 500 Ciba 1.43 5 Slip Ebecryl ™ 1360 UCB Chemicals 1.60 6Solvent Acetone Aldrich 68.26 Total 100

Example 3

Preparation of Pigment Dispersion in Dielectric Solvent

Polystyrene (0.89 grams, Polysciences, Inc., mw. 50,000) and AOT (0.094grams, American Cyanamide, sodium dioctylsulfosuccinate) were dissolvedin 17.77 grams of hot xylene (Aldrich). Ti-Pure R-706™ (6.25 grams) wasadded to the solution and ground in an attritor at 200 rpm for more than12 hours. A low viscosity, stable dispersion was obtained. Oil-blue N(0.25 grams, Aldrich) was added to color the dispersion. The suspensionwas then tested in a standard electrophoretic cell comprising two ITOconductor plates separated by a 24 microns spacer. High contrast,alternating white and blue images were observed with a switching rate ofabout 60 Hz and a rising time of 8.5 msec at 80 volts.

Example 4

Preparation of Pigment Dispersion

The experiment of Example 3 was repeated, except Oil Red EGN (Aldrich)and an electrophoretic cell with a 24 microns spacer were used. Highcontrast, alternating red and white images were observed with aswitching rate of 60 Hz and a rising time of 12 msec at 60 volts.

Example 5

Preparation of Pigment Dispersion

Ti-Pure R-706™ (112 grams) was ground by an attritor in a solutioncontaining 11.2 grams of a maleic anhydride copolymer (Baker HughesX-5231™), 24 grams of 3,4-dichlorobenzotrifluoride, and 24 grams of1,6-dichlorohexane (both from Aldrich). Similarly, 12 grams of carbonblack were ground in a solution containing 1.2 grams of alkylatedpolyvinylpyrrolidone (Ganex™ V216 from ISP), 34 grams of3,4-dichlorobenzotrifluoride, and 34 grams of 1,6-dichlorohexane(Aldrich) at 100° C. These two dispersions were then mixed homogeneouslyand tested. High contrast black and white images were observed with aswitching rate up to 10 Hz and a rising time of about 36 msec at 100volts.

Example 6

Sealing the Microcups by One-step Process

0.05 Ml of a UV curable composition comprising 1 wt % of benzyl dimethylketal (Esacure KB1™ from Sartomer) in HDDA (1,6-hexanediol diacrylatefrom Aldrich) was dispersed into, 0.4 ml of a dielectric solventcomprising 0.5 wt % of2,2,3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-nonadecafluoro-1-decanol(Aldrich) in FC-43™ from 3M Company. The resultant dispersion was thenimmediately filled into an array of microcups as prepared in Example 2.Excess of fluid was scraped away by a wiper blade. The HDDA solution wasallowed to phase separate for at least 30 seconds and cured by UVradiation (10 mw/cm²) for about 1 minute. A hard, clear layer wasobserved on the top of the microcups and the microcups were sealed.

Example 7

Sealing the Microcups by a Two-step (Overcoating and UV Curing) Process

The electrophoretic fluid prepared in Example 5 was coated onto themicrocup array as prepared in Example 2. A thin layer of Norland opticaladhesive NOA 60™ (Norland Products Inc., New Brunswick, N.J.) was coatedonto the filled microcups. Any excess of the UV adhesive was scraped offby a strip of Mylar™ film and cleaned by a piece of absorbing paper. Theovercoated adhesive was then cured immediately under a Loctite Zeta7410™ UV exposure unit for about 15 minutes. The microcups were sealedcompletely and no air pocket was observed. The thickness of curedadhesive layer was about 5-10 microns as measured by a Mitutoyothickness gauge.

Example 8

Sealing the Microcups by a Two-step (Overcoating and Moisture Curing)Process

The experiment of Example 7 was repeated, except the Norland adhesivewas replaced by Instant Krazy™ glue from Elmer's Products, Inc.,Columbus, Ohio. The overcoated adhesive was then cured for 5 minutes bymoisture in air. The microcups were sealed completely and no air pocketwas observed. The thickness of cured adhesive layer was about 5-10microns as measured by a Mitutoyo thickness gauge.

Example 9

Sealing the Microcups by a Two-step (Overcoating and InterfacialPolymerization) Process

The experiment of Example 8 was repeated, except the electrophoreticfluid was replaced by a 3,4-dichlorobenzotrifluoride solution containing0.3 wt % of tetraethylenepentamine (Aldrich) and the Instant Krazy™ gluewas replaced by an aliphatic polyisocyanate (Desmodur™ N 3300 from BayerCorp.) solution in anhydrous ether. A highly crosslinked thin film wasobserved almost immediately after overcoating. The dielectric solventwas completely sealed inside the microcups after the ether wasevaporated at room temperature. No air pocket was observed.

While the present invention has been described with reference to thespecific embodiments thereof, it should be understood by those skilledin the art that various changes may be made and equivalents may besubstituted without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation, materials, compositions, processes, process stepor steps, to the objective, spirit and scope of the present invention.All such modifications are intended to be within the scope of the claimsappended hereto.

What is claimed is:
 1. An electrophoretic display comprising a pluralityof cells where each of said cells comprises: 1) side walls, each of saidside walls having a top surface; 2) an electrophoretic dispersion filledtherein; and 3) a sealing layer which is in contact with both the topsurface of said side walls and said electrophoretic dispersion and iscurved.
 2. The display of claim 1 wherein said sealing layer has a sidethat is in contact with the electrophoretic dispersion, and that side iscurved.
 3. The display of claim 1 wherein said sealing layer has twosides and is curved on both sides.
 4. The display of claim 1 whereinsaid sealing layer is physically or chemically bonded to the top of theside walls of the cells.
 5. The display of claim 1 wherein each of saidside walls has an inner surface and said sealing layer is in intimatecontact with both the dispersion and the inner surface of the side wallsof said cells that are not covered by the dispersion.
 6. The display ofclaim 1 wherein said sealing layer extends over the top surface of theside walls of said cells.
 7. The display of claim 1 wherein said sealinglayer is a polymeric layer.
 8. The display of claim 1 wherein saidsealing layer is formed from a composition comprising a materialselected from the group consisting of polyvalent acrylate, polyvalentmethacrylate, cyanoacrylates, polyvalent vinyl, polyvalent epoxide,polyvalent isocyanate, polyvalent allyl, and oligomers or polymerscontaining crosslinkable functional groups.
 9. The display of claim 8wherein said sealing layer is formed from a composition comprisingpolyvalent isocyanate.
 10. The display of claim 8 wherein saidcomposition further comprises an ingredient selected from the groupconsisting of surfactants, antioxidants, initiators, catalysts,crosslinkers, thickeners, polymer binders, pigments, dyes and fillers.11. The display of claim 10 wherein said filler is silica, CaCO₃, BaSO₄,TiO₂, metal particles and their oxides or carbon black.
 12. Theelectrophoretic display of claim 1 wherein said sealing layer is formedfrom a sealing composition having a specific gravity lower than that ofthe electrophoretic dispersion.
 13. The electrophoretic display of claim12 wherein said sealing composition is a UV curable composition.
 14. Theelectrophoretic display of claim 12 wherein said sealing compositioncomprises a thermoplastic or thermoset precursor.
 15. An electrophoreticdisplay which comprises: a) two electrode plates; b) an array of cellssandwiched between the two electrode plates, where each of said cellscomprises: (i) side walls, the side walls having a top surface; (ii) anelectrophoretic fluid filled therein which comprises charged particlesdispersed in a dielectric solvent or solvent mixture; and (iii) apolymeric sealing layer which is in contact with both the top surface ofsaid side walls and said electrophoretic fluid and is curved towards theelectrophoretic fluid.
 16. The display of claim 15 further comprising anadhesive layer between said polymeric sealing layer and one of saidelectrode plates.
 17. The display of claim 16 wherein said polymericsealing layer is formed from a composition comprising a materialselected from the group consisting of polybutadiene, polyisoprene,styrene-butadiene copolymers, olefin rubbers, nitrile rubbers,polyvalent acrylate, polyvalent methacrylate, cyanoacrylates, polyvalentvinyl, polyvalent epoxide, polyvalent isocyanate, polyvalent allyl, andoligomers or polymers containing crosslinkable functional groups. 18.The display of claim 17 wherein said polymeric sealing layer is formedof a composition comprising polyvalent isocyanate.
 19. The display ofclaim 17 wherein said composition further comprises one or moreadditives selected from the group consisting of surfactants,antioxidants, initiators, catalysts, crosslinkers, thickeners, polymerbinders, pigments, dyes and fillers.
 20. The display of claim 19 whereinsaid filler is silica, CaCO₃, BaSO₄, TiO₂, metal particles and theiroxides or carbon black.
 21. The display of claim 16 wherein saidadhesive layer is a pressure sensitive adhesive, a hot melt adhesive ora heat, moisture, or radiation curable adhesive.
 22. The display ofclaim 21 wherein said adhesive layer is formed from a material selectedfrom a group consisting of acrylics, styrene-butadiene copolymers,styrene-butadiene-styrene block copolymers, styrene-isoprene-styreneblock copolymers, polyvinylbutyral, cellulose acetate butyrate,polyvinylpyrrolidofle, polyurethanes, polyamides, ethylene-vinylacetatecopolymers, epoxides, multifunctional acrylates, vinyls, vinylethers,and their oligomers, polymers, and copolymers.
 23. The display of claim22 wherein said adhesive layer is formed from a polyurethane, epoxide,multifunctional vinyl or acrylate.
 24. The display of claim 16 whereinsaid sealing layer and said adhesive layer are formed from the samematerial.
 25. The display of claim 24 wherein said same material is aradiation curable material.
 26. The display of claim 16 wherein saidsealing layer and said adhesive layer are formed from differentmaterials.
 27. The electrophoretic display of claim 15 wherein saidpolymeric sealing layer is formed from a sealing composition having aspecific gravity lower than that of the electrophoretic fluid.
 28. Theelectrophoretic display of claim 27 wherein said sealing composition isa UV curable composition.
 29. The electrophoretic display of claim 27wherein said sealing composition